ИСТИНА

Electrochemical oxygen reduction in aprotic media is a key process that determines the operation of advanced metal-oxygen power sources, e.g. Li-O2 batteries. The possibility of using carbon-based positive electrodes is still questionable due to the unclear ORR mechanism that comprises several chemical and electrochemical steps, as well as side reactions with carbon itself [1-3]. These steps may involve either soluble or adsorbed species, thus the nature of solvent and carbon electrode structure are key parameters that determine Li-O2 cell performance. It is known that in aprotic systems formation of superoxide species is the process that determines the ORR pathway and the nature of byproducts [4], however, the impact of carbon electrode structure on its interactions with superoxide species is still not completely understood. Here we reveal the role of carbon defects in Li-ORR elementary steps by means cyclic voltammetry of model carbon electrodes with different sp2-domain sizes (HOPG, glassy carbon, basal and edge planes of pyrolytic graphite), in DMSO-based electrolyte. We found that on nearly ideal sp2 carbon lattices (HOPG and basal plane of pyrolitic graphite) oxygen reduction to superoxide (O2 + e- à O2) and further superoxide reduction to lithium peroxide (Li+ + LiO2 + e- à Li2O2) could be seen on cyclic voltammogram as two separate electrochemical processes. We show that the first electron transfer step is outer-sphere process that does not involve oxygen adsorption, and does not depend on carbon electrode structure. On the contrary, the second electron transfer to form Li2O2 is strongly affected by the electrode surface, and could not be observed on the carbon surface with high amount of defects, that manifests the important role of solution-mediated Li2O2 formation. We also introduce the evidence that carbon surface defects are highly reactive towards superoxide species, resulting in enhanced amount of Li2CO3 being formed during ORR on the carbon surface with higher amount of defects.